Typical engine control requires control of fuel injection rate, intake air volume, ignition timing, and other operational parameters in response to output requirements for an engine. The fuel injection rate and intake air volume influence the air-to-fuel ratio, and the timing of ignition influences combustion efficiency and stability in a cylinder. Thus, the accuracy of calculating these control parameters significantly affects the output and emission performance of the engine.
A known calculating method used for such engine control is a differential estimation correction method. The differential estimation correction method extrapolates the slope of a variable actual intake air volume inhaled into a cylinder to estimate the subsequent intake air volume.
For example, Patent Document 1 (Japanese Laid-open Patent Application No. HEI-07-259621) discloses a technique which calculates the estimated value of the air volume inhaled into a cylinder, and then calculates the fuel injection rate on the basis of that estimated value for an engine control apparatus which injects fuel before the inlet stroke is finished.
This technique involves calculation of a difference between the latest intake air volume and the second latest intake air volume on the basis of a value detected by an air flow sensor which periodically detects the intake air volume for each engine rotation and estimation of the subsequent intake air volume through addition of the newest intake air volume to a value obtained by multiplying this difference by a predetermined estimation gain. Such a method can estimate an accurate intake air volume, and can prevent the variation of the air-to-fuel ratio and torque through the control of the fuel injection rate on the basis of the estimated intake air volume.
Unfortunately, in this predictive method, the accuracy of the estimated value may be decreased when the slope of the intake air volume varies. For example, FIGS. 4A, 4B are graphs showing examples of the variations of the actual intake air volume (actual air volume) and estimated value. In these graphs, the actual air volume for every stroke is calculated for a four-stroke four-cylinder engine wherein the estimated value is calculated two strokes before the actual air volume is calculated. The estimated value of the actual air volume is obtained by doubling the difference between the latest actual air volume and the second latest actual air volume (estimation gain is two) wherein the fuel injection rate is set on the basis of this estimated value.
FIG. 4A indicates a constant slope of the actual air volume, while FIG. 4B indicates a variable slope of the actual air volume. The horizontal axis of the graph represents time, in which vertical lines drawn for each half rotation (180 degrees) of a crankshaft indicate the boundaries of each stroke. The vertical axis of the graph corresponds to the air volume (the actual air volume, and the estimated value thereof). It should be noted that in the graphs, the thick solid line represents the actual air volume, the dashed line represents the estimated value thereof, and the thin solid line represents the actual air volume after two strokes.
Since the actual air volume does not vary from time S0 to time S2 in the graph, the estimated value is equal to the actual air volume. When the actual air volume varies after time S2, the estimated value is calculated according to the variation. At that time, if the variation of the actual air volume per unit time is fixed, the increment from time S2 to time S3 is equal to the increment from time S3 to time S4 and the actual air volume hereinafter increases at a fixed rate. As a result, a value obtained by doubling the variation of the actual air volume per stroke is substantially equal to an increment of the actual air volume after two strokes at time S2. In this way, the subsequent actual air volume can be estimated at a point two strokes before. In other words, as shown in FIG. 4A, the dashed line varies so as to follow the thin solid line after time S2.
On the other hand, as shown in FIG. 4B, in the case that the variation of the actual air volume per unit time is not fixed, a gap is caused between the estimated value and the actual air volume after two strokes, the accuracy of the estimation of the actual air volume depends on the slope of the actual air volume after the actual air is estimated.
For example, between time S2 and time S3 where the slope of the actual air volume increases, a value obtained by doubling the variation, between time S2 and time S3, is calculated as the estimated value regardless of a subsequent increase in the actual intake air volume; hence the dashed line goes below the thin solid line, and the estimated value becomes smaller than the actual air volume after two strokes. A shaded area between time S0 and time S3 corresponds to the shortfall of the air volume as the estimated value. On the contrary, the slope of the actual air volume decreases after time S3, and the dashed line goes over the thin solid line, so that the surplus of the air volume, shown by a shaded area, is included in the estimated value.
As described above, the conventional method in which the intake air volume is estimated on the basis of the slope of the actual air volume inhaled into the cylinder has a large estimation error. Unfortunately it is difficult to accurately estimate the intake air volume in a transient state in which the change rate of the intake air volume increases and decreases. The variation of the air-to-fuel ratio and torque caused by such a low estimation accuracy may result in low controllability of the engine.